Time-of-flight mass spectrometry for the measurement of molecular masses of compounds requires ionization of the molecules, which can be accomplished using a variety of techniques. Electrospray ionization (ESI) is a particularly advantageous ionization technique as it ionizes fragile molecules without fragmentation by generating highly-charged liquid drops of the analyte solution, which subsequently release ionized molecules of the analyte. Hence, ESI is referred to as a “soft ionization” technique that has enabled mass spectrometric measurement of organic ions and macromolecules, which would otherwise be difficult to ionize in the gas phase without substantial ion fragmentation.
ESI is particularly well suited for mass spectroscopy of proteomics and metabolomics. ESI is also suitable for certain analytes in ambient aerosol particles, though it has been difficult to utilize ESI in this application. Ambient aerosol particles are of considerable interest for mass spectroscopy due to their effect on human health and atmospheric visibility. They are also of interest due to their influence on radiative forcing, which is pertinent to the assessment of the global climate change. The chemical composition of such particles is difficult to analyze, as they persist in the atmosphere at mass concentrations of 10 micrograms per cubic meter or less in many environments. Nonetheless, analyses of particle chemical compositions have been attempted both in ambient field studies and in simulated laboratory environments via Aerosol Mass Spectrometers (e.g. the commercially available AMS from Aerodyne Research Inc. of Billerica, Mass.). In these instruments, particles are deposited on a surface, the surface is heated to evaporate the particle contents, and electron ionization (EI) is subsequently used to ionize the gas-phase molecules. EI is a “hard ionization” technique. The combination of thermal volatilization and EI results in fragmentation of large molecules, which cannot be measured directly using the current AMS devices. An alternative technique, the thermal desorption chemical ionization mass spectrometer (TDCIMS), utilizes chemical ionization (CI) instead of EI. However, direct measurement of completely unfragmented/unreacted species is not enabled.
ESI based mass spectroscopy for aerosol has so far been performed off-line, which involves collecting particles (submicrometer in size) for long periods of time onto filters and subsequently extracting and analyzing them in a laboratory. This process is labor-intensive, time-consuming, and precludes real-time measurements. In an attempt to develop a real-time technique facilitating ESI-like ion production for species within ambient aerosol particles, Grimm et al. (2006) showed that the ESI can be performed directly from droplets in the gas phase (i.e. field induced droplet dissociation), giving rise to ESI type ions directly from aerosol droplets. However, the technique is limited to large (>100 microns) liquid drops, prohibiting its application for measurements of smaller micrometer and submicrometer particles. Peng et al. (2007) demonstrated that proteins, introduced into the gas phase via matrix assisted laser desorption ionization (MALDI), could be uptaken into ESI generated droplets, and subsequently released from droplets as multiply charged ions through mixing an ESI generated droplet plume with a MALDI generated analyte plume (i.e. analytes were incorporated into droplets via droplet-analyte coagulation). Similarly, Shia et al (2008) showed that biomolecules (aerosolized via either laser desorption or by an ultrasonic nebulizer) could be collisionally incorporated into ESI like droplets, resulting in the eventual formation of multiply charged ESI-like biomolecular ions (these approaches are referred to as “extractive ESI”).
While these studies demonstrate the capture of aerosol particles by ESI generated droplets and subsequent ionization, unfortunately, these techniques are limited by collision kinetics between ESI droplets and aerosol particles. Calculations of particle collision rates demonstrate that gas phase based collision approaches require a high number concentration of aerosol particles needed to generate a sufficient number of ions for mass analysis. Hence, they are not suitable for ambient aerosol.
Recently, Horan et al. (2012) as well as Gallimore & Kalberer (2013) demonstrated that by colliding aerosol particles not with ESI generated droplets, but instead directly with the liquid cone of a stably operating electrospray, aerosol particles can be dissolved within the ESI solution and hence ionized. Because of the significantly larger collision length (for diffusive capture of aerosol particles) of the electrospray liquid cone as compared to droplets, such systems are a more promising route to the production of ESI-type, unfragmented ions from aerosol particles than either field induced droplet ionization or extractive ESI. However, the lower limit of detection is still high due to the poor collection efficiency.